In silver halide photography a photographic method, according to which a positive image is made without the use of a negative image or an intermediary process producing a negative image, is called a direct-positive method. A photographic light-sensitive material and a photographic emulsion for use according to such photographic method are called direct-positive material and direct-positive emulsion respectively.
A variety of direct-positive photographic methods are known. The most useful methods are the method, which comprises exposing prefogged silver halide grains to light in the presence of internal or external electron acceptors and developing them, and the method, which comprises subjecting a silver halide emulsion containing silver halide grains that have light-sensitive specks mainly inside the grains to an image-wise exposure and developing the exposed emulsion in the presence of a development nucleator.
With externally fogged direct-positive emulsions it is not easy to reach the high sensitivity level which can be obtained with internal latent image type direct-positive emulsions and which are developed by fogging development, e.g., by using hydrazines. This is probably due, at least partially, to the heterogeneous character of the external chemical fogging. However direct-positive emulsions show the advantage of not requiring a rather unecological processing as is the case when fogging developers are used. Especially in applications wherein copies are required from, e.g., radiological images of an X-ray diagnosis, the sensitivity of the externally fogged direct-positive emulsions used in such material is sufficient. Further an adequate and reproducible exposure latitude is required for a duplicating material. Because an accurate reproduction of an original image, without losses in image quality, neither in image density nor in image detail, is highly preferred, a direct-positive material with a low average contrast of about 1.0 is desired having a sensitometric curve showing an acceptable linearity without deviations or irregularities.
An improved layer arrangement in order to get a direct-positive element of extended latitude has been described in U.S. Pat. No. 3,647,463. However high amounts of silver coated in said layer arrangement cause a higher cost, a longer processing time, more silver recoverage and more environmental load.
Another attempt by making mixtures of at least 4 or 5 emulsions in one emulsion layer in order to controll the production is very disadvantageous with respect to the consistency in the manufacturing of direct-positive materials. Moreover an intensive controlling is required.
Therefor it is an object of the present invention to provide a direct-positive material having a low contrast and an extended latitude.
More specifically it is an object to provide a process for the reproducible and consistent manufacturing of such direct-positive material in order to make copies from radiological images.
Other objects will become apparent from the description hereinafter.